The mechanism by which complement activation at a cell surface results in prostaglandin synthesis and concomitant bone resorption in organ cultures is currently under investigation. This process appears to be dependent on immunoglobulins with a specificity for cell surface antigens and can be initiated by either the classical or alternative complement pathways. The interaction of the different immunoglobulin classes with the complement system and the influence of antigenic structure on the mode of complement interaction are under examination.